Chapter 13 Java AWT – Part II (Optional)

1. Chapter 13Java AWT – Part II(Optional) Lecture Slides to Accompany An Introduction to Computer Science Using Java (2nd Edition) by S.N. Kamin, D. Mickunas, E. Reingold

2. Chapter Preview In this chapter we will: • discuss the use of Java in animating World Wide Web applications • show how to write Java applets • introduce Java graphical components (e.g. Textfields, Buttons, and Labels) • show how Java programs can be made to react to user actions on Java graphical components • show how Java applications can be run from inside web browsers

3. Frames • A frame is a window with a title bar and a border • The Frame class is a subclass of Container class • Container class objects may have other components (e.g. Buttons) added to them using the add method • A typical Java GUI will create and display one or more frames • To make a frame visible the message setVisbible(true) must be sent to the frame

4. Layout Managers • Governs how components are arranged inside a Container object • The Container method setLayout allows the programmer to specify which layout manager (e.g. FlowLayout) is desired for a particular container object • The size of a Container object should be set explicitly by the programmer as well • Can use pack() to set size to accommodate preferred sizes of components)

5. Frame Example public class TwoButtons { Frame f; Button redButton, blueButton; public TwoButttons() { f = new Frame(“Two Buttons Frame”); redButton = new Button(“Red”); blueButton = new Button(“Blue”); f.setLayout(new Flowlayout()); f.add(redButton); f.add(blueButton); f.pack(); f.setVisible(true); } }

7. UML Notation • The filled diamond represents composition • This shows that the class TwoButtons contains a class Frame which contains a class Buttons • A good UML diagram only shows the classes and associations that are important to understanding the architecture • This UML diagram does not include super classes like Component or Container

8. Using Inheritance • It is possible to use inheritance to allow TwoButtons to become a frame in its own right • Note that the UML diagram will show TwoButtons as a subclass of Frame • You will need to use super()to set the frame title

10. Frame Inheritance Example public class TwoButtons extends Frame { Button redButton, blueButton; public TwoButttons() { super(“Two Buttons Frame”); redButton = new Button(“Red”); blueButton = new Button(“Blue”); f.setLayout(new Flowlayout()); f.add(redButton); f.add(blueButton); f.pack(); f.setVisible(true); } }

11. Other Simple Java Components • Label • contains some text that can be set using a constructor or the method setLabel • TextField • a box into which the user can type text • text can be edited (backspace, delete, etc.) • text can be retrieved using getText() • text can be set using setText()

12. Java AWT Event Model Including reactive program components involves: • Having the class header declare itself as implementing the ActionListener interface • Typically in the class constructor the class instance registers itself as being interested in listening for events from a newly created component • One method (e.g. actionPerformed) of the ActiveListener interface is defined

13. Frame Inheritance Example public class classname extends Frame implements ActionListener { … Button buttonname; … public classname() { … buttonname = new Button(“button label”); … add(buttonname); … buttonname.ActionListener(this); … } … public void actionPerformed(ActionEvent e) { … what to do when button is pushed … } }

15. UML Notation for Button Program • Abstract class indicated by italicizing the class name • Implements association (concrete class) indicated by line with a closed arrowhead • UML stereotype (constructors, accessor, interface implementation methods) indicated by enclosing a descriptive word inside guillements << >>

16. Closing the Window • You must implement the WindowListener and its seven required methods windowActivated, windowClosed, windowClosing, windowDeactivated, windowIconified, windowDeiconified, windowOpened • This is usually done in a programmer defined class like ClosableFrame and all classes needing these services would extend ClosableFrame

18. Using Conditionals with Reactive Components public void actionPerformed(ActionEvent e) { double fahr, cent; // check for user input in tFahr TextField if (e.getSource() == tFahr) { fahr = new Double(tFahr.getText()),doubleValue(); cent = 5.0 * (fahr – 32.0) / 9.0; tCent.setText(cent + “”); // update tCent TextField } else { cent = new Double(tCent.getText()),doubleValue(); fahr = 9.0 * cent / 5.0 + 32.0; tFahr.setText(fahr + “”); }

19. Checkboxes • Used to allow user to select one or more items • Always has a label • Program often needs to react to the user selection(s) • Declaration example: Checkbox powerBrakes = new Checkbox(“Power Brakes”); Checkbox powerSteering = new Checkbox(“Power Steering”); Checkbox ac = new Checkbox(“Air Conditioning”); … add(powerBrakes); add(powerSteering); add(ac);

20. Programming Checkboxes • Program is declared with implements ItemListener • Checkbox is registered by calling addItemListener • Event is handled using itemStateChanged argument type ItemEvent • The ItemEvent argument is used to tell which item triggered the event by calling getSource

21. Radio Buttons • A group of Checkboxes in which only one item can be selected at a time • Implemented using a Java CheckboxGroup • Items are declared initially as selected (true) or unselected (false) • Example: CheckboxGroup gender; Checkbox maleCheck = new Checkbox(“Male”, gender, true); Checkbox femaleCheck = new Checkbox(“Female”, gender, true);

22. Processing Radio Buttons public void compute() { … boolean female = (gender.getSelectedCheckbox() == femaleCheck); … if ((bodyMassIndex > 27.8) || (female && (bodyMassIndex > 27.3)) lHigh.setText(“This is considered high”); else 1High.setText(“this is not considered high”); }

23. Drawing in a Frame • To draw in a Frame you need to the override the frame’s paint method: public void paint(Graphics g) • Graphics objects are defined by the Java runtime system and are used for drawing operations • The Frame should be considered to be a grid with upper left coordinates (0,0) and positive coordinates (x,y) for the lower right

24. Typical Drawing Code public class MyDrawing extends ClosableFrame { … public MyDrawing() { … } … public void paint(Graphics g) { … g.drawLine(…); … g.drawImage(…); … } }

25. Repaint and Update • The paint method requires an argument and actionPerformed does not know what to supply when called • The method repaint will clear the Frame and then call paint along with supplying the needed missing argument • You can prevent clearing the Frame when using repaint by overriding the method update before calling repaint

27. Panel and Canvas Classes • The Panel class is container subclass that is used to reserve a rectangular portion of a Frame to place other components • The Canvas class is not a Container subclass, but does allow you to reserve a portion of a Frame to draw in

28. Comparing Layout Managers FlowLayout • Default frame layout • Components are placed on new line only when needed GridLayout • Frame is declared as a grid of fixed size (e.g. two rows and three columns) • Components are placed in the grid left to right and top to bottom BorderLayout • Frame is divided into north, south, east, west, and center • Components are placed by the programmer in the desired location using the add method

30. Typical GUI Frame Panel p1 = new Panel(); p1.setLayout(new GridLayout(2,1)); p1.add(component); p1.add(component); Panel p2 = new Panel(); p2.setLayout(…); p2.add(component); … setLayout(new BorderLayout()); add(“North”, p1); add(“Center”, p2);

31. Scrollbars • The leftmost position of a horizontal scrollbar corresponds to some integer value and the rightmost position corresponds to a larger integer value • Scrollbars can be displayed vertically • User movement options • Clicking either end button causes bubble to move in unit increments • Clicking the are between bubble and end button causes movement in 10 unit increments • Clicking and dragging the bubble in the desired direction

32. Hypertext Markup LanguageHTML – Skeleton <HTML> <HEAD> <TITLE>Page Title</TITLE> </HEAD> <BODY> Text and graphics to display in browser </BODY> </HTML>

33. HTML Text Displays • Displays a centered heading in browser window <CENTER> <H1>Naploean I</H1> </CENTER> • To identify text paragraphs to be displayed <P> In my spare time I like to do origami and spend quiet evenings at home with my wife Josephine. </P>

34. HTML Using Pictures and Links • Displays a centered picture <CENTER> <IMG SRC=“jojo.jpg”> </CENTER> • To display a text link to another web page <A HREF=“jojo.html”>Josephine</A> • To display a picture link to another web page <A HREF=“jojo.html”><IMG SRC=“jojo.jpg”></A>

35. Applet Tag and HTML • To include a Java applet as part of an HTML document <APPLET CODE=“BodyMass.class” WIDTH=0 HEIGHT=0> </APPLET> • Java applet shell for BodyMass.class import java.applet.*; public class BodyMass extends Applet { … public_html void start() { new BodyMass(); } }

36. Transforming a Java Program into an Applet • Set the size you want the applet to be in the HTML file • Add the import statement to the Java code java.applet.*; • Change header from extends Frame to extends Applet • Change constructor heading to public void start(); • Eliminate any Java code that is meaningless in applets